Large quantities of fly ash are a by-product when coal is combusted to produce energy in power plants. In the United States alone, the annual production of fly ash is on the order of tens of millions of tons. Significantly, as coal-burning power plants are becoming increasingly relied upon to meet this country's growing energy needs, fly ash production is certain to further increase.
Although fly ash is useful and recyclable in its raw form for a limited number of purposes, including as a bed in road construction, fly ash supply exceeds demand and currently most of the fly ash produced is disposed of as waste. Consequently power plants frequently incur substantial landfill fees for disposing of their unsellable, untreated fly ash. It should, therefore, be appreciated that a need is identified for developing useful markets for fly ash. This will provide the dual benefit of preserving landfill space and reducing utility operating costs.
Toward this end it should be appreciated that fly ash that is low in residual carbon content is an excellent cement additive. Recent clean air regulations have, however, focused upon reducing the. nitrogen oxide (NO.sub.x) emissions of electric utility company power plants. As a result, many of these plants now use low NO.sub.x burners. These burners have the unfortunate side effect of increasing the residual carbon content of the fly ash above acceptable levels allowing use of that fly ash as a cement additive. Accordingly, the residual carbon must be removed from the fly ash if the market for fly ash as a cement additive is going to be able to continue to be tapped.
It should also be appreciated that the residual carbon in the fly ash is a high grade product that may be reburned by the coal-fired utilities. Accordingly, if the residual carbon is recovered in an efficient manner, it may be used for recombustion and energy production. Consequently any effective process for separating residual carbon from fly ash provides the multiple benefits of increasing energy production, preparing the fly ash for marketing as a cement additive, reducing or eliminating fly ash disposal costs and preserving landfill space. The present invention proposes utilizing froth flotation to recover the residual carbon from the fly ash and achieve these important benefits of increased economic efficiency and improved resource utilization.
Froth flotation was first discovered in 1906. It was developed for the non-ferrous minerals industry to recover extremely fine, free minerals from slime. This technique, developed nearly 90 years ago, remains basically the same today. The froth flotation mechanism employs the principles of colloid chemistry, crystallography and physics. Separation of one mineral from another is achieved by the use of specific reagents and chemical conditions. The addition of chemical reagents makes one mineral surface hydrophobic through absorption, while leaving the other mineral surfaces hydrophilic. Benification is accomplished by aerating the slurry or suspension, so that air bubbles become laden with hydrophobic particles and rise to the surface of the pulp or slurry, leaving behind the hydrophilic particles. Of course, it should be appreciated that froth flotation is a complex Physico-chemico-mechanical process. The process and, particularly, bubble-particle attachment is influenced by many variables including pH, pulp or slurry density, particle size, bubble size and air flow.
While others have tried to use froth flotation in the past to separate and recover residual carbon from fly ash, they have met with only limited success. This is primarily because prior art efforts have not determined the proper mixture of reagents to promote effective and efficient separation and recovery.
Examples of prior art approaches include U.S. Pat. No. 4,426,282 to Aunsholt which discloses a relatively complex, multi-step method of separating coal or carbon particles from fly ash by flotation at pH levels as low as pH 3-5. The Aunsholt method utilizes a collector, a frother and a dispersant. Preferably, the collector is a mineral oil fraction predominately containing C.sub.5-10 hydrocarbons. Preferably, the frother is a terpene oil or a cresylic acid and the dispersant is preferably a polyglycolether.
The U.S. Pat. Nos. 5,047,145 and 5,227,047 to Hwang disclose additional methods of recovering carbon from fly ash using fuel oil as a collector in combination with both magnetic separation and flotation techniques.
Notwithstanding these prior efforts, the removal and recovery of carbon from fly ash using conventional froth flotation remains largely an inefficient process. This inefficiency arises in large part because of the excessive amounts of reagents required to render carbon particles hydrophobic. This is likely due to the fine size of the carbon particles as well as their very large surface area. Additionally, methods of flotation in the prior art require a significant amount of flotation time in order to achieve substantial flotation of the carbon particles from the fly ash. Further, carbon flotation systems in the prior art have also proven inadequate in the recovery of significant amounts of higher grade carbon. Thus, a need is identified for a more efficient and effective method of flotation adapted to separate residual carbon from fly ash providing enhanced carbon recovery.